The field of art to which the invention pertains includes elevators, and more specifically portable elevators.
Portable elevators, such as lift trucks, which are adapted for loading both from the front of a telescopic upright and from one or both sides thereof, herein sometimes referred to as "side loading lift trucks," frequently work in very narrow aisles formed by rows of storage racks with barely sufficient side clearance for truck travel. Some such lift trucks are designed so that the upright mast structure may turn about a vertical axis with the load always carried in front of the upright. My invention applies not to the rotating upright type but to those in which the upright is fixedly secured to the truck frame whereby the upright must handle loads both forwardly and to the side thereof.
Conventional lift truck uprights of the front loading type only, are not designed to carry the heavy side loads required of such side loading trucks. Heavy side loads may damage conventional uprights, causing excessive wear rates, premature upright guide roller failures, and sometimes cause elements of the mast structure to bind or "hang-up" while other elements and the hydraulic lift cylinder are lowering. It is important that the various upright elements be able to travel vertically in both directions without sticking or binding.
Heavy side loads also cause conventional uprights to bend and flex excessively in the direction of the side load, particularly at high lift heights. Currently, load lift heights of 480 inches are not uncommon, while lateral deflections of uprights must be minimized for side loading lift trucks. The telescopic members of the mast also wear and spread apart, and even if seizure of the telescopic members do not occur, the mating parts soon have a sloppy relationship with each other. Excessive side deflection also results in lowering the lifting capacity of the truck.
With the advent of side loading lift trucks it became apparent that uprights must be designed for handling heavy side loads at relatively high elevations with minimum side deflection. A number of different design measures have been employed to achieve the required upright rigidity. Among design techniques currently in use are the following:
(1) X-bracing of the various upright sections; PA0 (2) The addition of side thrust rollers at various locations in the upright; PA0 (3) Various forms of cross-over lift chains and supporting cross-over structure. In this regard see U.S. Pat. No. 3,830,342 which discloses relatively complex chain-sprocket arrangements wherein a load carriage or upper mast section is suspended from chains which extend laterally across the truck via sprockets carried by an intermediate mast section and which are tied to a lower mast section. Also see U.S. Pat. No. 3,782,503 which discloses a type of cross-over structure for transferring a portion of a side load to the opposite side of the upright through side mounted stabilizer chains and a transverse torque shaft, and my U.S. Pat. No. 3,716,158 which uses vertical racks and pinions mounted on torque shafts to operatively connect the sides of the upright to the lift carriage for a similar purpose; PA0 (4) The use of additional retainment between upright rails so that the various telescopic members thereof are not extended as much as is common in conventional uprights; PA0 (5) The incorporation of a third set of upright rails oriented for off-setting lateral forces and deflection; PA0 (6) Employing hydraulic lift cylinders at both sides of the upright structure rather than at the center as in conventional uprights; and PA0 (7) Straightening and machining the upright rails and imposing exacting tooling and manufacturing tolerances.